1. Field of the Invention.
The present invention relates to electrically conductive enclosures for protecting sensitive electronic components from ambient electromagnetic fields and, more particularly, to arrangements for maintaining electrical continuity between such enclosures and associated doors, hatches, covers and the like which provide access to the interior of the enclosures.
2. Description of the Prior Art.
In many applications utilizing high technology electrical or electronic equipment, the sensitivity of such equipment to electromagnetic interference (EMI) mandates the installation of such equipment within a metal shielding enclosure to safeguard the equipment during operation in an electromagnetic field environment. It will be recognized that the reduction or virtual elimination of such electromagnetic interference becomes an all encompassing consideration, particularly in aerospace and military applications where failure to safeguard the electronic equipment from such electromagnetic interference would render the equipment inoperative, or possibly induce false signals causing erroneous information to be provided by the equipment.
Such electronic equipment is therefore typically installed into an electrically conductive metallic enclosure which will function as a shield against electromagnetic interference, with the typical enclosure being sealed by the installation of a door panel onto an opening in the enclosure, thereby providing access to the interior for service or maintenance. It will be recognized that the enclosure may require other apertures therein for electrical connections, cooling devices, or input/output interfaces for use with the equipment. Closing the one or more openings in the enclosure with a conductive door panel or panels may not ensure adequate shielding of electrical equipment contained within the enclosure because of the likelihood of loss of circuit continuity between the door panel and the enclosure. Even an extremely narrow aperture between the door panel and the enclosure or surface resistance from corrosion will interfere with the effectiveness of the enclosure as an electromagnetic shield.
The solution generally embraced by the art has been the use of a conductive gasket between the door panel and the enclosure, the conductive gasket being required to compressively deform as the door panel is fastened onto the enclosure to establish a pressure contact circuit path between the door panel and the enclosure. Such gaskets have included metallic wool or fabric, spring-loaded fingers, deformable curved gaskets made of thin spring stock, and tubular mesh gaskets, all of which may be flattened between the door panel and the enclosure to maximize electrical conductivity through the gasket material between the door panel and the enclosure. One of the more effective types of conductive gasket materials is the wire mesh type, which is also known as "knitted wire", and which is sufficiently resilient to firmly contact both the surface of the door panel and the surface of the enclosure when the door panel is fastened to the enclosure. Examples of such gaskets are shown in Johnson U.S. Pat. Nos. 3,424,853 and 4,247,737. Generally, such wire mesh or knitted wire gaskets do provide a continuous electrical contact between the access door panel and the enclosure which is effective for a limited period of time after installation. The problem with the knitted wire gaskets of the prior art stems primarily from the fact that the metals contained in such conductive gaskets are rarely compatible, galvanically, with the metals required for structurally sound enclosures.
Moreover, such wire mesh gaskets are particularly limited in effectiveness when used in a corrosive environment, such as a sea environment which would be encountered by installation of such equipment near the ocean, on an ocean-going vessel, or in aircraft based on a carrier. The metals contained in such conductive gaskets are attacked by galvanic action more rapidly in such a marine environment where they encounter moist salt air, salt spray and the like so that the effectiveness of the shielding provided by such enclosures is rapidly degraded.
A temporary solution to this problem is to utilize water repellent oil as a corrosion inhibitor by spraying or soaking the conductive gasket with oil before the door panel is fastened to the enclosure. Unfortunately, while this protective measure is initially quite effective in preventing corrosion, in only a relatively short time thereafter the oil must be replenished or corrosion will occur. Thus, this approach requires that the door panel be removed and the gasket be re-oiled periodically to provide effective protection against corrosion. Alternatively, if a rubber gasket is used in addition to the conductive metallic wire mesh gasket, there is a possibility that the conductive gasket may be oiled without removing the door panel from the enclosure; however, it is apparent that providing such periodic maintenance is a burden and it would be highly preferable to have a system affording longer protection without maintenance.
Historically, technology exists for providing oil from a reservoir to a location over an extended period of time. Such technology is displayed in Berryman U.S. Pat. Nos. 2,065,558 and 2,068,530, which disclose automobile battery clamps incorporating a small reservoir of oil with a wick utilized to draw the oil from the reservoir to the battery post. It will be immediately recognized that providing such reservoirs would be both expensive and impractical, inasmuch as the conductive gasket may be fairly lengthy depending on the size of the enclosure and the door panel. Thus, the system taught by Berryman is felt to be inadequate. It is therefore apparent that there exists a substantial need for a conductive gasket or similar element which will be corrosion-resistant and which will not require maintenance for extended periods of time. Such a conductive gasket must be rugged, easily maintained, and effective and reliable over extended periods of time. The gasket must be flexible, and should not interfere with sealing the door panel against the enclosure. Finally, it is desirable that all of the above features and advantages should be provided at minimal cost.